I wear a heart rate monitor when I work out. You program it so it has your height, weight and target heart rate range. Not only does it let me know when I'm slacking off it counts the calories I burn according to my body and heart rate. I'm sure it's not 100% accurate but it's not too far off. There are some really high tech ones out there and more simple ones, I love mine so if you're interested I would recommend you look into getting one.

I use a heart rate monitor as well and have found it to be much more on the money than machines or even the sites that have calculators on them

Mine uses body weight, resting heart rate, height and sex as well as my actual heart rate. Its good because it takes into account my increasing fitness (lowering weight AND decreasing resting heart rate) - the fitter you are the harder you have to work to burn those pesky calories lol. Your body becomes more efficient which is why sites generally aren't all that accurate or machines in the gym.

I am in the market to buy a new one for myself cause the above one is my partner's and he is getting back into running and wants it back lol. Damn him :P

but if you are after a site try www.calorieking.com.au and use their tools. Its alright for a general number out.